The present invention relates to a two-cycle engine with a scavenge pump for feeding air into the engine and with a direct fuel injection system, and more particularly to a system for controlling the timing of opening and closing scavenge and exhaust rotary valves in the engine.
In two-cycle engines, each cycle is completed with one revolution of a crankshaft so that self-intake operation into a cylinder is not carried out. For this reason, a method is used wherein air is induced into a crankcase with the use of vacuum generated in a crankcase while a piston is in its upward strokes, and wherein intake into the cylinder is carried out through a scavenge port by increased pressure in the crankcase due to downward strokes of the piston while the gases produced after combustion in the cylinder is forced out of the cylinder by the scavenging flow of the intake gas. Further, for control depending upon the load on the engine, a throttle valve is provided in an intake system, and the amount of intake is adjusted by varying the degree of opening of the throttle valve.
In such a known system, the scavenging flow tends to be insufficient under low load condition due to a small amount of intake, and this causes misfiring with resultant unstable operation of the engine. Moreover, it is difficult to obtain a smooth torque characteristic of the engine in accordance with operating conditions so that much fuel is consumed and exhaust gas is degraded. Furthermore, there is a limit to increase in the scavenging ability of the engine with such a type of scavenging system using the crankcase pressure. This means that it is difficult to obtain a sufficiently increased torque of the engine under high load.
Under such circumstances, it has been desired to obtain increased scavenging ability and to reduce flow of the fuel through the cylinder. With a view to realizing the above, it has been proposed to use a scavenge pump for increasing the scavenging pressure and to inject the fuel directly into the cylinder chamber by means of a fuel injector so as to prevent the unburnt fuel from flowing through the cylinder during the scavenging.
A two-cycle engine provided with a scavenge pump and a fuel injector is disclosed in, for example, Japanese Utility Model Laid-Open (unexamined) Publication No. 62-97,229. The two-cycle engine disclosed in this publication is provided with overhead valves as in the case of a four-cycle engine and with a supercharger in an intake system and an exhaust blower in the exhaust system. The supercharger and/or the exhaust blower are operated depending upon the operating condition of the engine.
In the known two-cycle engine described above, low scavenging efficiency accompanied by the overhead valves must be compensated for by the supercharger and the exhaust blower, so that the engine necessarily has a complicated construction as well as increased loss of power for driving auxiliary mechanisms of the engine. Furthermore, in the known engine, timing of opening and closing the intake and exhaust valves is always fixed, so that it is not possible to increase the scavenging efficiency and the output of the engine in accordance with the engine operating conditions.